Corrosion, particularly of ferrous metals, in crude oil producing, collection and refining systems is a significant problem. The adverse effects of corrosion which shorten equipment life and increase downtime have multiplied as the refinery process has expanded and become more complex. Corrosion problems in a refinery operation can be the result of any one or a combination of the components found in the crude oil, the chemicals used in the refinery process, and the process conditions.
In many hydrocarbon refining streams there is an aqueous phase present therein. The aqueous phase is simply water entrained in the hydrocarbons being processed and/or water added to the system for purposes such as stripping. Oftentimes contaminants are present in the aqueous phase and they contribute to the problems of corrosion in various refinery processes.
With sour crudes, an additional corrodent is H.sub.2 S, either originally present in the sour oil or gas and/or formed at processing temperatures by decomposition of sulfur compounds in the charge stocks. Other contaminants such as NH.sub.3 and HCN are found in alkaline waters downstream of fluidized catalytic cracking units (FCCU), cokers and other hydroprocessing units, and in alkanolamine units.
The presence of cyanide can promote hydrocarbon damage of carbon steels exposed to aqueous sour steams. The inhibition of corrosion caused by cyanide and sulfur compounds is of great interest in the refining industry.
However, treatment programs are not readily determined due to industry problems with accurately measuring free cyanide in samples containing sulfide. Typically, the ratio of H.sub.2 S to HCN is from 10:1 to 1000:1 in these systems. Sulfide must be removed from the sample at the time of collection to prevent the potential reaction of cyanide with sulfur oxidation products, i.e., polysulfide. If sulfide is left in the sample, it can be oxidized by entrained oxygen to polysulfide which can react with cyanide and lead to erroneous results. Sulfide also interferes with current cyanide measuring methods such as the Selective Ion Electrode and trying to remove the sulfide by, for example, precipitation as a metal sulfide salt, can also remove a significant amount of cyanide as an insoluble metal cyanide complex making a determination of cyanide concentration difficult.
The present inventor has discovered a method for determining the concentration of cyanide in an aqueous system where sulfide and thiocyanate are present. The present invention measures the conversion of cyanide to thiocyanate by fixation of samples through two separate procedures. This measured difference in thiocyanate levels allows for determining cyanide concentration without the concern of H.sub.2 S levels and the inherent dangers in handling cyanide.